In the high-pressure process for preparing melamine, generally urea and ammonia are reacted at temperatures between about 320 and 450° C. and pressures between about 50 and 600 bar to form liquid melamine and off-gas principally consisting of ammonia, carbon dioxide and small amounts of gaseous melamine and other minor components. After the melamine melt has been separated off from the off-gas, this melamine melt is worked up by various processes for preparing pure melamine, while the off-gas is preferably recirculated to a urea plant.
Before recirculating the off-gas to the urea plant, however, this off-gas must be freed from the melamine content and the other minor components, since these are unwanted in the urea plant.
According to U.S. Pat. No. 3,700,672, the off-gas is brought into contact with fresh urea in countercurrent in a single stage at temperatures between 135 and 250° C. and approximately at the synthesis pressure of the melamine reactor, that is to say at 50 to 200 bar, and in this process is freed from the melamine present and the other minor components. The melamine and the minor components in the off-gas are absorbed in the fresh urea melt, thus recovered from the off-gas and then recirculated back to the melamine reactor. As a result the melamine yield in the melamine synthesis reactor can be increased. In the process employed according to U.S. Pat. No. 3,700,672, the maximum operating temperature for the off-gas scrubbing is 250° C. This upper temperature limit is required according to U.S. Pat. No. 3,700,672, since above 250° C., solid by-products are formed which interfere in the recycling of the urea into the melamine synthesis reactor, and gaseous by-products are formed which are unwanted in the off-gas.
The disadvantage of said process is that as a result of the single-stage contacting of off-gas and urea melt, the exit temperatures both of the purified off-gas at the scrubber head, and of the urea melt enriched with melamine and other off-gas minor components in the scrubber bottom phase must be approximately the same. To maintain the required low operating temperatures in the off-gas scrubber, it is necessary, therefore, to remove a large amount of the heat introduced into the scrubber with the hot off-gas as waste heat via a cooler. Although this waste heat can be used to form steam, in the energy balance of the melamine synthesis process, it denotes an energy loss, since the urea melt exiting from the scrubber at a maximum 250° C. must be reheated in the melamine reactor to the synthesis temperature of about 380° C. This means that the loss of energy in the scrubber must be compensated for by feeding heating energy to the synthesis reactor.
A further disadvantage of low operating temperatures in the scrubber bottom phase is the formation of by-products. These are those substances which form exothermically in the scrubber, that is to say with release of heat, from the ammonia and the carbon dioxide of the off-gas, for example carbamate and water. As a result of formation of these substances, heat is produced in the scrubber at a low temperature level, which must be removed as waste heat. These by-products must later be decomposed in the melamine reactor into the starting materials ammonia and carbon dioxide at a high temperature level by supplying heating energy. This energy transport in the form of chemical energy from the synthesis reactor to the scrubber is a great loss.
The requirements of an optimum off-gas scrubber are therefore many: firstly, the off-gases coming from the melamine reactor are to be freed as completely as possible from the melamine and the other minor substances, secondly, the energy efficiency of the melamine synthesis process is to be improved by better utilization of the off-gas heat. The object thus posed is to find a process which takes these requirements into account.